Life is nothing without structure, and Rokugan life is filled with structure. Prior to even your birth, karma, fate and the wheel of destiny have been long spinning, and now you must fulfill it! All men and women are born under one of these castes.
In order of significance:

The Emperor, the divine child of Amaterasu, the Sun Goddess
The Emperors family
The Imperial Clan - Hantei, Seppun, Otomo
The 7 Daimyo of the Major Clans - Crab, Crane, Dragon, Lion, Phoenix, Scorpion, Unicorn
The Lords of the Clan Families - See Clans Listing for full details
The Lords of the Minor Clans - Mantis, Fox, Sparrow, Wasp
Samurai of the Major Clans (Bushi & Shugenja)
Samurai of the Minor Clans (Bushi & Shugenja)
Ronin - Masterless Samurai.
Heimin "Half-People"- Farmers, craftsmen, artisans, merchants.
Hinin "Non-People" - Geisha, torturers, morticans, Eta